This invention relates to electrical communication multiplex systems, and more particularly to such systems of the interrogator-responder type which automatically respond to preselected conditions at protection zones. Such systems may be used to monitor fire, burglar, holdup and supervisory conditions, for example. The system may find other applications, including those in which any particular condition may be defined by a particular status of an electrical circuit. Examples of such other uses include the detection of high water or the change of conditions of a patient in a hospital when such conditions may be represented by temperature, respiration rate, or other measurable factors. Thus, the uses for the present invention are virtually limitless, and it is expected that the applications of the present system will be those in which a relatively rapid indication of an alarm condition or change of status is desired.
It is the usual practice in multiplex systems for monitoring a plurality of protection zones to provide one central station from which a source of interrogation signals originates to be supplied to a plurality of transponders or responders which reply according to the condition of the particular protection zones with which each transponder is associated. The common supply of interrogation signals requires that the transponders distinguish among the interrogation signals according to each interrogation signal addressed to a particular zone, so that only a response from an addressed protection zone will be supplied to the central station. Consequently, the other transponders associated with the non-addressed protection zones must reject all interrogation signals other than those interrogation signals intended to evoke a response. In this manner, the central station may interrogate and receive information about the status of each protection zone from their associated transponders as each is addressed and replies in turn. Although such known multiplex systems may appear unduly complex, it is advantageous in that only one central station providing the source of interrogation signals is required, since the single central station will provide a central and rapid indication of an alarm status or change of status from which authorized personnel such as fire persons and police persons may be dispatched in response to an alarm condition.
Known prior art multiplexing systems used to accomplish the foregoing general practice of monitoring a plurality of protection zones are relatively complex and expensive. Usual prior art systems involve the use of pulse width modulated or pulse code modulated signals from the central station to address or designate certain and individual transponders associated with particular protection zones for activation and response. Since the transponders are addressed by pulse width or pulse code interrogation signals, they must respond with similar pulse width modulated reply signals. Consequently, both the central station and each remote transponder must include the relatively complex circuitry necessary to detect, decode, transmit and receive information by the modulated signals. Such complex systems, of course, necessarily involve significant expense.
Known pulse width and pulse code modulation multiplex alarm systems generally require very high grade electrical conductors connecting the central station with each remote transponder, due to the necessity of maintaining the high frequency modulated interrogation and reply signals without significant degradation. These communication paths are generally special telephone lines and the lease cost or tariff rate of high grade telephone lines suitable for maintaining high frequency signals at high information transmission rates is relatively expensive. Alternatively, if the electrical conductors are to be avoided, radio communication paths may be established, but the radio transmitters and receivers involve significant expenses too. Thus, because of the relatively complex nature of signal employed, the prior art systems require relatively costly communication paths between the central station and the remote transponders.
Pulse code or pulse width modulation signals are generally employed by prior art multiplex systems to avoid false signals which might occur as a result of response by transponders associated with non-addressed protection zones or by spurious electrical signals such as those that result from electrical transients caused by lightning, switching or interruption of service. Owing to the complex nature of the modulated signals used in the prior art systems, spurious signals generally have little effect, but this avoidance is possible only through the use of the complex and expensive circuitry and system design of such modulated systems.
It is the general object of this invention to provide a multiplex interrogation system using pulses for monitoring protection zones which avoid the foregoing deficiencies of the prior art and which provides superior performance relative to known multiplex systems at significantly lower cost than such known systems.
It is an object of this invention to provide a multiplex interrogation system which determines the alarm status of a protection zone from a response pattern comprised of replies over a predetermined number of prior sequential interrogations of that protection zone.
It is another object of this invention to provide a multiplex interrogation system for monitoring protection zones which employs relatively simple circuit elements in the central station and in the transponders.
It is a further object of this invention to provide a multiplex interrogation system operating with relatively non-complex signals for interrogation and reply.
It is another object of this invention to provide a multiplex interrogation system which is compatible with and functions very effectively with low cost communication paths such as low grade telephone lines.
It is another object of this invention to provide a multiplex interrogation system which significantly reduces the amount of communication necessary between the transponders and the central station to enhance the performance of the system and reduce the cost.
It is another object of this invention to provide a high amount of information communication from use of an unmodulated pulse signal.
It is still a further object of this invention to provide information communication under conditions highly immune to spurious signals and other false signals while still providing a rapid response to a genuine alarm condition.
It is still a further object of this invention to provide a multiplex interrogation system which will continue to provide information even if some of the transponders become disassociated with or disconnected from the electrical paths connecting them to the central station.
It is a further object of this invention to provide ready recognition of alarm conditions at the central station, to provide a record of the alarm conditions, and to provide a classification of priority of the relative importance of various alarm conditions if multiple alarms arrive at the central station in close time proximity.
To achieve these and other objects as well as further advantages, one embodiment of the present multiplex interrogation system may monitor a large number of protection zones over a single pair of inexpensive and low-tariff DC grade telephone wires. The protection zones may be utilized for many functions including fire alarm monitoring, burglar alarm monitoring, holdup alarm monitoring, or supervisory condition monitoring. Each protection zone may be assigned any type of function. The system may utilize DC pulses for interrogation and reply of information transmittal. The central station addresses each protection zone in sequence by sending out a positive interrogation pulse on the telephone line. The number of interrogation pulses sent in each scan of all zones is designated "n". Each transponder counts the interrogation pulses supplied by the central station, and responds only to the particular interrogation pulse which corresponds to the count assigned to that particular protection zone monitored by its associated transponder, each protection zone having a different numerical assignment between the numbers 1 and "n". The response of the transponder is a reply pulse immediately following the interrogation pulse according to the condition of the protection zone. Addressing an interrogation pulse to the protection zone and the return of the reply pulse from the transponder associated with the protection zone addressed after receipt of the interrogation pulse consecutively occurs until each protection zone has been interrogated. Interrogation of every protection zone comprises one scan of all of the protection zones of the entire system. At the termination of each scan, the system resets and another scan is made. As the system is continually scanned, a response pattern of the reply pulses over a predetermined number of scans for each protection zone, is developed and the response pattern determines the condition or status of each zone. In a preferred embodiment, four scans are required to determine the status of a protection zone. The response pattern of each protection zone over a predetermined number of scans makes available a number of status conditions, for example, a set condition, an alarm condition, or an out-of-service condition. The response pattern also essentially eliminates any effect that spurious signals on the telephone line might cause.
The response pattern is updated on each new scan, and in this manner changes of status of the protection zones are rapidly available. When a change of status has occurred, information stored at the central station is supplied according to the protection zone which has undergone a change in status. This information may include the protection zone number, the function of that zone, and the status to which the protection zone has changed. This information is available until an operator overseeing the system acknowledges the change of status and takes action accordingly. Information stored at the central station and the response pattern reduces the communication between the central station and the transponders to a minimum. The information also is used to determine the relative priority between a number of rapidly occurring alarm conditions. Some alarm conditions such as fire and holdup are designated as priority alarms, and in the event that a priority alarm status is determined at approximately the same time as a non-priority alarm status is determined, the central station has a provision for distinguishing between priority and non-priority alarms so that the priority alarm information is provided in precedence over the non-priority information.
The present interrogation multiplex system also has provision by which an operator at the central station can inquire about the alarm status of any protection zone and may receive information about that zone. Information of protection zones available for use or currently out of use due to broken wires or other similar disconnection in the communication path is also available. This system also includes means to synchronize the addressed interrogation of the protection zones and the reply of the transponder associated with the addressed protection zone.
A fuller understanding of the invention as to its organization, method of operation and practice, and further objects and advantages may be obtained by reference to the following brief description of the drawings and detailed description of the invention.